1) Field of the Disclosure
The disclosure relates generally to structural parts and the production of structural parts, and more specifically, to hat stiffener configurations and methods used in forming stiffened panel structures such as used in aircraft.
2) Description of Related Art
Composite and metal structures are used in a wide variety of applications. In aircraft construction, composites and metals are typically used to form the fuselage, wings, tail section and other aircraft components. For example, aircraft fuselage may be formed of stiffened panel structures having skin panel structures constructed of composite or metal to which stiffening elements, such as stringers, may be coupled to improve the strength, stiffness and stability of the skin panel structures. The stringers may be bonded or attached to the skin panel structures and may be configured to carry various loads. In addition, aircraft may incorporate stiffening elements, such as stringers, in the wings, where the stringers may be coupled to the skin panel structures in order to form upper and lower surfaces of the wings.
Stiffening elements, such as stringers, may be provided in a wide variety of cross-sectional shapes. For example, a stringer may comprise a hat stiffener made of a metal material or a plurality of composite plies and having a cross-section with a hat-shaped configuration. The hat stiffener may have a cap, a pair of webs where each web extends from opposite ends of the cap, and a pair of flanges where each flange extends outwardly from a base portion of each web. The flanges may facilitate coupling of the stringer to the skin panel structure.
The design of stiffened panel structures, such as those reinforced with hat stiffeners, may be limited in performance and/or weight efficiency by the stability of the webs of the hat stiffener. For example, to achieve greater bending stiffness of the stiffened panel structure, it may be beneficial to offset the cap of the hat stiffener farther away from the skin panel structure. However, as the webs of the hat stiffener which support the cap get taller, they may become more prone to instability, such as by buckling. Thus, web stability of the hat stiffener may be a limiting factor in the design and optimization of stiffened panel structures, such as those reinforced with hat stiffeners.
Methods and systems exist for designing and sizing stiffened panel structures reinforced with hat stiffeners. For example, one known method includes limiting the height of the webs of the hat stiffener. However, the bending efficiency of the stiffened panel structure may be related to the relative height of the webs of the hat stiffener. Therefore, by limiting the height of the webs, potential bending efficiency may, in turn, be limited.
Another known method for designing and sizing stiffened panel structures reinforced with hat stiffeners includes increasing the thickness of the entire hat stiffener or increasing the thickness of the webs of the hat stiffener. However, increasing the thickness of the entire hat stiffener or even just the webs of the hat stiffener may not be the most weight efficient method for generating buckling or crippling resistance, such that the end result may be a design with a considerable strength margin but with a minimal or zero stability margin. Such design may result in increased overall weight of the stiffened panel structures which may, in turn, result in increased fuel costs and decreased vehicle efficiency.
Further, another known method for designing and sizing stiffened panel structures reinforced with hat stiffeners includes decreasing the cap angle, that is, the angle formed between the cap and each web, which may increase the steepness of each web. Decreasing such cap angle may also decrease the length of each web necessary to achieve a particular offset of the cap in relation to the skin panel structure of the stiffened panel structure. However, this may be less effective as the cap angle approaches 90 degrees. Moreover, as the webs become steeper, the shear path between the cap and skin panel structure may be compromised which may, in turn, lead to radius opening issues if the stiffened panel structure is loaded in shear.
Accordingly, there is a need in the art for an improved hat stiffener and method that provide advantages over known designs and methods.